July 28, 2011

Numerical supremacy of modern humans over Neandertals

I haven't read this paper yet, but, as I pointed out recently, the numerical superiority of a population A over another B does not necessarily mean that B went extinct; it could just as easily have been fully absorbed.

This paper refers to modern humans and late Neandertals in France, so it is not entirely relevant to modern-Neandertal admixture, which, if it occurred, must have taken place in Asia, to account for the relative uniformity of Neandertal admixture across Eurasians.

Nonetheless, if modern humans outnumbered Neandertals when that admixture did take place, then this would give us a way to estimate whether it was sporadic or commonplace. For example, a 4% Neandertal admixture and a 10:1 modern/Neandertal population ratio would suggest that about "half" the Neandertals were absorbed, and admixture was commonplace. If, on the other hand, the two species had similar population numbers during contact, then the low Neandertal admixture estimates are consistent with sporadic, uncommon admixture events.

Tenfold Population Increase in Western Europe at the Neandertal–to–Modern Human Transition

Paul Mellars, Jennifer C. French

Abstract

European Neandertals were replaced by modern human populations from Africa ~40,000 years ago. Archaeological evidence from the best-documented region of Europe shows that during this replacement human populations increased by one order of magnitude, suggesting that numerical supremacy alone may have been a critical factor in facilitating this replacement.

10 comments:

The relative uniformity of Neandertal admixture across Eurasians could also mean that Neandertals and Eurasians had a common ancestor or that gene flow amongst Eurasians is much more fluid than between sub-Saharan Africans.

I think the evidence is reasonably convincing that Neanderthals had become considerably inbred by the time 'modern' humans entered Europe. That may explain a lower breeding success and their consequent 'replacement'.

Recent studies have shown that Neanderthals disappeared too soon following the modern human dispersals to the Neanderthal territories during the early Upper Paleolithic, so I think the Upper Paleolithic mental/cultural revolution in modern humans is the biggest cause of the Neanderthal extintion.

This is all very fascinating, but I have a question concerning the semantics here and the conceptualizations behind them. Since it is clear that there was gene flow from Southwest Asian Neandertals into Eurasian populations of Paleolithic Homo sapiens sapiens (and as a consequence of this, traces of this gene flow also exist in East Africa and in New World “African” populations), why still refer to Neandertals as a separate species? It seems clear to me that they belonged to the same specific lineage as AMH, and thus were a very distinct subspecies of Homo sapiens. The simple fact is that gene flow between populations, and the normal reproduction of fertile offspring, takes precedence over physical morphology in the defining and taxonomic classification of species. And anyway, I don’t think anyone is seriously willing to assert that Eurasians and some Africans are inter-specific hybrids.

About ancient admixture sapiens/Neanderthal, one important fact must be taken into consideration: the admixture has been detected in present people, not ancient Homo sapiens. Therefore the low admixture in today Europeans can be explain with the low contribution of the first European Homo sapiens population to today Europeans. Multiple migrations have taken place since 30 000 BC (notably with the discovery of agriculture in Middle East around 10000 BC). Consequently, the gene-flow from west Eurasia has been very important and the dilution of Neanderthal contribution could have been equally important in ancient Europeans. The answer to homo sapiens/Neanderthal admixture in Europe lays in ancient Homo sapiens genomes.

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